The Amihan Glider Project

Amihan is the Northeast trade wind in the Philippines. It is the summer prevailing winds that give you a steady onshore flow that is great for beach soaring or wind surfing on the east facing coastlines. The Habagat is the Southwest monsoon that brings the rainy season.

The Amihan glider concept developed from daydreaming of soaring the warm winds on the beaches of Negros Occidental Philippines. As the wind comes over the beach it rises up over the sand dunes and other obstacles creating a narrow band of lift that a glider could take advantage of. The beach often provides a convenient launch and landing area. The pilot who is skilled enough to fly within the confines of the standing wave of lift and the strand can fly all day and land anytime for lunch or rest, finds himself in a heaven of low and slow flight in some of the most beautiful places on Earth.

This idyllic mode of flight also has some pit falls which the Amihan project strives to overcome: low and slow.... these are things the dangers of which have been drilled into every pilot since crashing became a thing. Dune soaring combines slow flight, low altitude and turning to stay in the lift band. Ordinarily this is a recipe for disaster because flying close to the stall speed and then turning can quickly put you into the dirt.

With the original dune soaring gliders being flexi wing hang gliders with forgiving stall characteristics, low stall speeds and a prevailing wind that is often higher than the gliders stall speed gives the skilled pilot the ability to turn without steep banking and being almost stationary over the ground. Control was by weight shift and thereby no air disturbances from control surfaces. The Amihan is rigid wing glider with control surfaces although still foot launch able. This presents some challenges: after launching by foot you will be airborne without your feet on the rudder pedals meaning that you will initially not have any rudder yaw control. Ailerons provide banking but simultaneously cause adverse yaw – bank left and yaw right. To mitigate these issues ailerons were replaced with a spoiler on each upper surface near the wing tip as the left spoiler will drop the left wing and cause a bit of drag to yaw the wing in the direction of the turn and the same on the right, allowing the pilot to turn without using rudder. After launch the pilot puts his feet on the rudder pedals which can then be used for slips and coordinating turns.

There is still the issue of stalling during low speed turns. At first wing slots and stall fences were selected, but even though slots do mitigate tip stalls and soften stall behaviour, they are passive. After some thought and observation of the Helio Courier, slots were replaced with aerodynamically controlled slats, two per wing for the entire span. This creates active stall mitigation and allows full length Fowler flaps which become important for low speed flight when deployed to 10 degrees, and lowers landing speeds when at 25 and 40 degrees.

The challenge for a rigid glider is performance range. Rigid gliders usually fly a lot faster than flexi wings, so bringing the stall speed down to 31mph clean and 15mph with flaps coupled with a 15mph wind brings the glider back into the manageable range for beach soaring and foot launch.